Water is a basic necessity for supporting life on this planet. Some regions of the world are fortunate to have adequate or abundant sources of fresh water due to a combination of moisture-rich, regional climate and the available natural resources, such as lakes and rivers. Other regions, such as the Middle East, are not so fortunate. Lack of abundant annual rains and the arid climate make fresh water a precious commodity.
Ironically, more than half the earth is covered by water, but that is in the form of seawater (SW), which is harmful to land-based life and plants if used in its natural state due to its high salinity. To capitalize on this abundant resource, desalination plants and systems process SW, as well as brackish water (BW) having a low salinity compared to seawater, into a safer, consumable form.
In recent years, boron in drinking and irrigation waters has become an important issue. Boron naturally exists in SW at concentrations of about 4 to 6 mg/L, with a maximum of about 7 mg/L in some waters. Toxicological effects of human exposure to excess boron, potentially from the consumption of water produced by desalination, have been reported. Boron is also an essential trace element for plant and crop growth and is contained in fertilizer. However, boron has been linked to detrimental effects on some plants and crops. This impacts the use of desalinated water for irrigation, where boron levels above 0.3 mg/L can potentially lead to foliage damage and reduction of fruit yield of some sensitive fruits, such as citrus and kiwis. As a consequence, the World Health Organization (WHO) recommends a guideline of 0.5 mg/L of boron in drinking water. The European Union (EU) has also classified boron as a pollutant of drinking water (EU Council Directive 98/83/Ec: 1 mg/L) if the boron concentration is at least 1 mg/L. At the end of 2008, WHO proposed to regulate boron concentration to be below 2.4 mg/L. However, the required boron concentration value in product water of each desalination plant depends on the system design of the plant, the usage of water, and the policy of the country.
In order to comply with the regulations and recommendations noted above, most desalination plants strive for the lowest concentration of boron to meet agricultural demands because if the desalinated product water is safe for irrigation, it will naturally be safe for human consumption. However, current conventional systems cannot reach this goal without incurring prohibitive costs in additional equipment and operational expenses.
For example, it is noted that boron in aqueous solutions exists as boric acid, B(OH)3, and or borate anion, B(OH)4−, based on the pH of the solution. It is well known that boron compounds in seawater do not dissociate to ions at low or natural pH. At lower seawater pH, the major species is boric acid in molecular form. Due to the smaller size and the absence of ionic charge in the molecular form of boric acid, this results in lower membrane rejection. At higher seawater pH, membrane rejection increases strongly due to a shift to the charged form B(OH)4−, i.e., B(OH)4− has a larger molecular size and a negative charge. Unfortunately, increasing seawater pH can lead to increasing the potential precipitation of large amounts of alkaline scale deposits. In the case of boron rejection by conventional RO (reverse osmosis) membranes, the rejection is affected by pH, permeate flux, temperature and salt concentration. The boron rejection of current RO membranes at nominal test conditions is about 85-90%. This corresponds to about 78-80% boron rejection with permeate boron concentration range of 0.8 to 1.3 mg/L in the operation of commercial SWRO (seawater reverse osmosis) systems. Thus, the extremely low boron concentration of ≦0.4 mg/L as per the above regulations cannot be achieved by a single pass RO operation.
Some alternative options include SWRO followed by a three-stage BWRO (brackish water reverse osmosis) with pH change, SWRO followed by boron selective ion exchange resin (BSR), and SWRO followed by a hybrid process of BSR and BWRO. All the above-mentioned techniques require additional capital and operational costs compared to the single-pass RO.
Another option attempts to optimize a single-pass RO process at feed pH of 9.5-10 to reject boron at higher levels and comply with regulations. This requires introducing new antiscalants to control alkaline scales due to seawater having a high scaling potential. In addition, it is expected that this type of process requires a higher caustic consumption rate, e.g., around 100 mg/L at seawater feed pH≦10, due to the high calcium bicarbonate concentration in seawater. However, no such antiscalants are known to the inventor.
In light of the above, it would be a benefit in the art of desalination to provide a system and/or method for removing boron to extremely low levels with minimal economic impact. Thus, the removal of boron from saline water using alkalized NF membrane pretreatment solving the aforementioned problems is desired.